Analytical measurements of particle dispersions frequently require dilution in order to eliminate particle-particle interactions as an element in the analysis. For example, in light scattering measurements, it is desirable to operate in the regime in which single-particle scattering approximations are valid. It is also desirable to operate within the linear range of the analytic instrumentation, typically at concentrations in the order of milligrams of particulates per gram of diluent, for performing static and dynamic light scattering spectroscopy and particle counting. If a sample is too concentrated, some analysers will not work properly.
The method of Ultrasound Spectroscopy has been recently developed for particle characterisation because it demonstrates linearity of the attenuation spectrum to higher concentrations than light scattering. Nevertheless the concentrations achieved are still not high enough for all slurries to be measured in-line and thus some dilution is often desirable or essential.
The corrections for particle-particle interactions involve approximations of complex real particle behaviours and thus an imperfect correction. Thus there remains an advantage for dilution even where particle-particle corrections are available, since it avoids the need for introducing these approximations.
Methods for the characterisation of dispersions containing, for example, micrometer and submicrometer-size particles, are important in understanding particulate systems in general. While detection systems have been disclosed for performing measurements on such dispersions, the sampling problem remains to be addressed effectively. A number of disadvantages exist in the prior art methods and apparatus. For example, difficulties have been observed in obtaining representative samples. Additionally mixing times of the sample and diluent in order to ensure uniform distribution of the sample throughout the diluent, and the time required for the measurement tend to be large and consequently the sampling and measuring process can be unwieldy and inefficient.
A further problem observed with the prior art is that often an entire sample is diluted and fed through an analyser. This type of batch analysis results in a substantial amount of diluent being required and accordingly, a large reservoir volume in which a homogenised dispersion of particles in diluent takes place.
U.S. Pat. No. 5,907,108 (hereinafter referenced as '108) discloses a system and method for sampling and dilution of homogenous particle dispersions. Sample and diluent are combined by a metering process in which the dilution ratio is controlled by the relative flow rates as determined by pumps. The combined sample and diluent are mixed by the use of a mixing conduit where the sample is extruded by the pump into a passing stream of diluent, also pumped. The mixing of sample with the dispersant occurs slowly in a conduit which requires stirring aids such as baffles to achieve full mixing. The dilution ratio of a single stage is limited only by the metering ability of the pumps.
The dilution is further increased by resampling the output of one dilutor with another, the 2nd and subsequent stages all having their dilution ratios each controlled by the flow rates of pumps. At least two pumps, or pump heads, are required per stage of dilution. Further, in the '108 patent exemplification the flow rates of sample are relatively very low and residence times within the dilutor very high, for example, typically on the order of a minute. Additionally, the '108 patent exemplification design is very difficult to clean or unblock. This may only be achieved by automatic sequence control of the various pumps with its associated complexity.